On common knowledge
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There’s a funny thing about ‘common sense’ and ‘common knowledge’. We often use them interchangeably but, they are not quite the same.
Common sense is that supposedly self-evident wisdom you can appeal to when you don’t want to argue: ‘it’s just common sense.’ Politicians love it. So do op-ed writers. The problem is common sense is highly subjective and, as The New Yorker puts it, it can be highly weaponised to justify one person’s nonsense.
Common knowledge is different and more interesting. In game theory, it means not only that everyone knows something, but that everyone knows that everyone knows.
Take the Emperor’s New Clothes fable. When the child points out that the Emperor is naked, he is not adding to people’s knowledge by pointing out something they already knew, but rather adding to their knowledge by letting them know that everyone else knows.
That recursive loop is what turns private belief into public certainty. As The New Scientist explains, common knowledge is the glue of social coordination, from markets to moral norms.
This is the logic behind the Super Bowl ads, according to The Economist:
Advertisers and spin doctors deftly exploit common knowledge…the value of a Super Bowl spot is not just that millions will see it; it is also that viewers know that millions of others have seen it
It’s the publicness that gives it force. The ad becomes a cultural reference point, a shared wink everyone can nod to at the water cooler the next day.
Take the Hermès Birkin: 20 years ago it was the epitome of quiet luxury - if you knew, you knew. Now, you can’t move for influencer posts flexing their Birkins, Kellys and other variations. While Hermès still retains its exclusivity, the appeal has started to wane because everyone knows that everyone knows what a Birkin is. It is now very loud luxury.
The same mechanics drive protests, market bubbles and even fashion trends. It’s not enough for people to believe something privately. They need to know others believe it too.
[Common knowledge] also provides a logical explanation for all sorts of seemingly rational behaviour: economic bubbles and manias, deliberately indirect and vague speech, social media storming mobs.
However, social media also fractures shared awareness. We can no longer assume that ‘everyone knows’ anything. We live in partial echo chambers, little information cocoons, where it feels like a belief is universal when it may just be local. We may believe the Emperor is fully clothed because we have seen 1,000 Tiktoks saying that he is whilst a different person’s feed is showing the complete opposite.
Two Substacks highlight the pros and cons of common knowledge: describes the impact of when teams rely on assumed shared knowledge that may not really exist and summarises Pinker’s case that common knowledge is what turns murmurs into movements.
So next time you hear the phrase ‘it’s just common sense’, pause. Ask yourself: is this really self evident? Or is it trying to smuggle in the authority of common knowledge - the sense that everyone knows that everyone knows?
Because when that recursive loop really kicks in, that’s when empires, markets and emperors themselves can unravel overnight.